1. Field of the Invention
The present invention relates to a NOx sensor diagnostic device that diagnoses abnormality of a NOx sensor disposed in an exhaust passage of an internal combustion engine, and an exhaust gas purifying system using the device.
2. Description of Related Art
An exhaust gas purifying system for detecting NOx concentration in an exhaust passage based on an output value of a NOx sensor disposed in the exhaust passage of an internal combustion engine is conventionally known (see, e.g., JP-A-2003-120399 and JP-A-2002-047979).
In such an exhaust gas purifying system, abnormality of the NOx sensor due to deterioration, failure or the like needs to be diagnosed in order to detect the NOx concentration with high precision. In JP-A-2003-120399, the concentration of NOx discharged into the exhaust passage is forcibly fluctuated by fluctuating displacement or temperature of exhaust air circulated in a combustion chamber, or ignition timing of fuel more greatly than normal control. If the resultant variation of the output value of the NOx sensor is shifted from a range that the output value can cover when the NOx sensor is normal, it is determined that the NOx sensor is abnormal.
In JP-A-2002-047979, abnormality of an exhaust emission control device including the NOx sensor is detected by determining whether the output value of the NOx sensor corresponds to the amount of NOx emission of 0 (zero), in an operational state in which an amount of NOx emission from an internal combustion engine is estimated to be 0 (zero) in a fuel-cut state, for example.
However, in JP-A-2003-120399, while the abnormality of the NOx sensor is being diagnosed, the displacement or temperature of exhaust air circulated in the combustion chamber, or the ignition timing of fuel is repeatedly changed more greatly than the normal control. As a result, the engine operation condition fluctuates more greatly than the normal control during the abnormality diagnosis of the NOx sensor.
By the method of diagnosing the abnormality of the NOx sensor by determining whether the output value of the NOx sensor corresponds to the amount of NOx emission of 0 (zero), in an operational state in which the amount of NOx emission from the engine is estimated to be 0 (zero), such as in JP-A-2002-047979, it is difficult to distinguish between normal and abnormality of the NOx sensor with high precision because the level of the output value is low even if the NOx sensor is normal.